Trigeminal Mediated Headshaking Syndrome in Horses: Causes, Diagnosis & Treatment

Trigeminal Mediated Equine Headshaking (TMHS) is a condition in horses characterized by involuntary, repetitive, and often violent head movements. This neurological disorder can significantly impact a horse’s comfort, performance, and quality of life.

TMHS results from dysfunction or hyperactivity of the trigeminal nerve. This nerve is responsible for sensation in the face and head, and its overactivation can lead to pain or discomfort, prompting the horse to shake its head. ^[1]

The success of treatment for headshaking varies depending on the severity and individual response. While some horses respond well to management strategies, others may experience chronic symptoms requiring ongoing care.

Read on to learn more about trigeminal-mediated headshaking, including how to recognize the signs and seek prompt veterinary assistance if you notice changes in your horse’s behavior or movement.

Headshaking in Horses

Headshaking in horses is a repetitive, tic-like behavior that can be triggered by disorders or injuries related to the head or neck, riding techniques, and other causes. ^[2]

Horses typically move their heads up and down, though some may toss their heads side-to-side or exhibit rotary movements. In some cases, the behavior may even become violent. ^[3]

While occasional headshaking is considered normal, frequent headshaking is abnormal and warrants further investigation. ^[2]

Abnormal headshaking is often mistaken for a horse reacting to insects around their face. However, horse owners frequently seek veterinary evaluation when the behavior becomes recurrent or starts interfering with riding and their horse’s well-being. ^[3]

Severe or recurring equine headshaking may be idiopathic, meaning it has no identifiable underlying cause. However, most cases are currently believed to be associated with dysfunction of the trigeminal nerve, a condition known as Trigeminal Mediated Equine Headshaking Syndrome (TME headshaking or TMHS).

Trigeminal Mediated Equine Headshaking Syndrome

Trigeminal Mediated Equine Headshaking Syndrome (TMHS) is a neuropathic condition in horses characterized by abnormal activation or dysfunction of the trigeminal nerve, leading to chronic, involuntary headshaking behavior. This syndrome is presumed to result from heightened sensitivity or reduced activation thresholds of the trigeminal nerve, which transmits sensory signals from the face.

TMHS is typically associated with signs of pain localized to the muzzle and nose, triggered by environmental stimuli such as light, touch, noise, or odors, and can significantly affect the horse’s behavior and performance. ^[3][4]

Headshaking may also be triggered by exercise, which can increase airflow through the nasal passages, blood flow to the nasal and turbinate mucosa, and the intensity of the maxillary arterial pulse. ^[3] Additionally, researchers have observed headshaking triggered by activities such as eating and procedures involving nasal swabs. ^[4]

TMHS has received increased attention in recent decades as a major cause of headshaking in horses. The role of the trigeminal nerve in headshaking was recognized more than a century ago by a veterinarian who described infraorbital neurectomy as an effective treatment. ^[3]

Interest in the trigeminal nerve as a cause of equine headshaking grew due to parallels with similar conditions in humans. Disorders involving this nerve in people cause sudden shooting or burning sensations across the face. This intense, sharp pain, often described as “lancinating,” follows one or more pathways of the trigeminal nerve. ^[3]

In humans, TMHS is divided into two syndromes: ^[3]

• Classical: No specific cause is identified other than vascular compression
• Symptomatic: A causative lesion is detected

The current hypothesis is that damage to the protective covering of the trigeminal nerve alters nerve signal conduction, leading to facial pain. ^[3]

Trigeminal Nerve Anatomy

The trigeminal nerve is the largest cranial nerve, playing a crucial role in both sensory and motor functions. It transmits sensory information from the face, muzzle, and nasal passages to the brain and controls the muscles involved in chewing. ^[4]

The trigeminal nerve complex is a sophisticated system composed of the following components: ^[4]

• Trigeminal Nerve: The main nerve responsible for transmitting sensory and motor signals.
• Trigeminal Ganglion: A collection of sensory nerve cell bodies responsible for processing pain and temperature signals.
• Three Primary Branches: The ophthalmic (V1), maxillary (V2), and mandibular (V3) nerves, each serving distinct regions of the face and head.
• Terminal Branches: Smaller nerves that extend from the primary branches to specific areas, such as the muzzle and nasal passages.
• Central Components: Sections of the trigeminal nerve within the spine and brainstem that process and integrate sensory input.

The sensory components of the trigeminal nerve are key to its function. The ophthalmic, maxillary, and mandibular branches relay sensory information, such as pain, temperature, and touch. The trigeminal ganglion, located at the base of the skull, serves as a hub for these signals. ^[4]

One of the primary branches, the maxillary nerve, has several significant sub-branches, including:

• Zygomaticofacial
• Pterygopalatine
• Major palatine
• Minor palatine
• Caudal nasal
• Infraorbital nerves

Dysfunction or hypersensitivity in any part of this system can result in the pain and abnormal behaviors associated with Trigeminal Mediated Equine Headshaking Syndrome (TMHS). While involvement of the trigeminal nerve in headshaking was suspected for many years, it has only been definitively confirmed within the last decade. ^[1]

Risk Factors

The average age of onset for TMHS in horses is typically between 8 and 10 years, though it can occur anywhere from 4 to 17 years. All horse breeds are susceptible to developing this disorder, but Thoroughbreds and geldings may be at a higher risk for reasons that remain unclear. ^[5][6]

Causes

Researchers are not entirely certain why the trigeminal nerve becomes sensitized in horses affected by Trigeminal-Mediated Headshaking (TMHS).

In humans, the herpes virus is known to cause neuropathic pain, but this does not appear to be the case in horses. Instead, researchers theorize that TMHS is a functional abnormality of the trigeminal nerve rather than a structural one, suggesting that a successful treatment may be achievable. ^[6]

Since headshaking is most often seen in geldings and can be seasonal, one hypothesis is that the condition is caused by increased activity of gonadotropin releasing hormone (GnRH). This leads to increased circulating concentrations of luteinizing hormone (LH) and follicle stimulating hormone (FSH). These hormonal shifts may lead to seasonal changes in trigeminal nerve excitability in affected geldings. ^[3]

Researchers have proposed that the way TMHS is sometimes triggered by light exposure may be similar to the phenomenon of “photic sneezing” in people, where intense sunlight induces sneezing. ^[4]

Photic stimulation via the optic nerve can lead to a tickling sensation in the nasal mucosa. This phenomenon may occur due to intense light stimulation of the optic nerves, resulting in cross-activation of the maxillary branch of the trigeminal nerve. ^[4]

However, another theory, known as “parasympathetic generalization”, suggests that parasympathetic nerves adjacent to the branches of the trigeminal nerve are co-activated. Similarly, neural pathways could be involved and triggered by exposure to bright light. ^[4]

It is important to distinguish TMHS from headshaking caused by musculoskeletal pain. Lame horses may exhibit headshaking behaviors, particularly when ridden. These behaviors may include moving the head up and down, side to side, or tilting it to one side. ^[5]

The exact cause of TMHS remains unclear. Recent research has investigated the potential involvement of dietary cation-anion balance and cecal microbiota in this syndrome. However, more studies are needed to further explore the role of dietary factors. ^[1]

Symptoms

Symptoms of TMHS most often occur in spring and summer, suggesting that symptoms may be triggered not by the intensity of daylight, but by increased daylight hours. However, some owners report that their horses exhibit headshaking signs year-round. ^[3][6][7]

The signs of TMHS are consistent with neuropathic pain of the trigeminal nerve. Horses with TMHS typically display headshaking in a vertical manner, which may be violent. This behavior often includes sharp vertical flicks and signs of nasal irritation. ^[6]

Horses may also rub their muzzles or strike at their muzzle with their forelimbs. They may snort or act as if an insect has flown up their nostril. ^[5]

Headshaking symptoms are usually worse during exercise, particularly at a trot. As the condition progresses, horses may begin to show symptoms even when at rest. ^[3][5]

Other symptoms of TMHS include: ^[3][5]

• Excessive snorting or sneezing
• Tearing of the eyes
• Flipping of the upper lip
• Anxious facial expression
• Nasal discharge
• Rubbing the nares or side of the face on the lower foreleg, a post, or the ground

Headshaking can become so severe that the horse is unsafe to ride or even handle. ^[3]

Interestingly, some horses experience periods of symptom remission that can last from weeks to years. The cause of remission is unknown, but long-term remission rates are generally low. ^[4]

Diagnosis

Headshaking syndrome can often be confirmed based on symptoms alone. However, further diagnostics may be required to identify the underlying cause and determine effective treatment.

Common diagnostic approaches that your veterinarian may take include: ^[8]

• Physical exam
• Bloodwork
• Neurologic exam
• Ophthalmologic exam

Accurate diagnosis of TMHS requires a detailed history of the affected horse and close observation of clinical signs, including potential triggers. Ruling out other causes of headshaking is essential, but this process often involves a lengthy elimination approach. ^[5][7]

Veterinarians may also perform additional examinations of the horse’s ears and mouth. An upper airway endoscopy, including the guttural pouches, may be conducted to rule out alternative causes of headshaking or facial pain. ^[4]

Other diagnostic tools that may assist in diagnosing or ruling out TMHS include:

• CT scan
• Maxillary nerve block
• Pain assessment

CT Scan

If other assessments are inconclusive, a contrast tomography (CT) scan of the head may help identify the underlying cause of headshaking. CT scans are particularly valuable for ruling out disorders affecting the bones and soft tissues of the head. ^[4][8]

Maxillary Nerve Block

Maxillary nerve blocks can be administered by a veterinarian to confirm the presence of facial pain in horses. ^[8] By anesthetizing the maxillary nerve on both sides of the face, a diagnosis of TMHS may be established.

This procedure requires sedation and physical restraint to ensure safety for both the horse and the veterinarian. ^[4] Video recording the horse before and after administering the nerve block is recommended to objectively assess the horse’s response.

Unfortunately, complications are common with this procedure, including: ^[4]

• Hematoma
• Worsening of head shaking
• Infection
• Neuritis
• Painful neuroma formation

Pain Assessment

Humans with neuropathic pain report symptoms ranging from tingling to unbearable electric, shock-like pain, which can significantly diminish their quality of life. Researchers assume that the pain associated with TMHS may be similar in horses. To better understand and manage this condition, several grading systems have been developed to quantify the severity of headshaking, which may correlate with the level of pain. ^[6]

The Talbot system is a grading scale that primarily evaluates the impact of headshaking on a horse’s behavior. Horses are scored as follows: ^[6][9]

• 0/3: No headshaking
• 1/3: Mild signs during exercise
• 2/3: Obvious signs without behavioral changes
• 3/3: Severe signs, including noticeable behavioral changes

The Roberts system focuses on the utility or rideability of the horse. A score of 1/3 constitutes mild signs and no interference of ridden exercise. A score of 3/3 constitutes headshaking even at rest. ^[6][9]

The Newton system is a combination of the Talbot and Roberts systems, focusing on impaired rideability but also behavioral changes. Horses scored 1/5 show mild clinical signs and facial muscle twitches but are still rideable. On the other hand, horses classified as 5/5 are dangerous with severe behavioral changes. ^[6][9]

It’s important to note that the Talbot, Roberts, and Newton scales are all based on subjective evaluation. This means they are prone to observer bias. ^[9]

The pain grimace scale may be the most objective scale available for determining severity of TMHS. This scale uses six facial expressions involving the ears, eyes, chewing muscles, mouth, and nostrils. The pain grimace scale uses rating points from 0 (no pain present) to 2 (pain clearly present). ^[9]

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Treatment & Management

With TMHS in horses, the pathology and exact cause remain unclear, making effective treatment a challenge. Furthermore, even when horses share the same underlying condition, their response to treatment can vary widely. ^[6] Owners should also be aware that there is a high failure rate of treatments for horses with TMHS. ^[4]

That said, the following treatments and management strategies have been successful for some horses with TMHS:

• Medications
• Nose nets
• UV blocking
• Adjunct therapies
• Nutritional supplementation
• PENS therapy
• Surgery

Medications

Cyproheptadine, an antihistamine with anti-serotonergic properties, was one of the first drugs used for headshaking syndrome in horses. This drug has been successful in some horses with photic triggers of headshaking. However, carbamazepine combined with cyproheptadine may be a better option. ^[3]

Other medications that have been used in affected horses include: ^[3][4]

• Non-steroidal anti-inflammatory drugs
• Corticosteroids
• Gabapentin
• Alpha-2-agonists
• Fluphenazine
• Phenytoin
• Phenobarbital
• Melatonin
• Sodium cromoglycate eye drops

Unfortunately, medical therapy doesn’t appear to be completely effective at resolving headshaking and improvements are often short-lived. ^[3]

Some medications may also cause drowsiness, making horses unsafe to ride or handle. Many of these drugs are also not allowed for use during competition. ^[6]

Nose Nets

Nose nets are thought to help reduce headshaking by providing continual sensory stimulation to the face and muzzle. The nose net is an affordable, noninvasive treatment option that is risk-free. Nose nets are also allowed at some equestrian competitions. ^[3][4][6]

In research studies, up to 75% of horse owners report improvements in their horse using either a full net, which covers the muzzle and upper and lower lips, or a half net, which covers only the nostrils and upper lip. ^[4]

UV Light Blocking Strategies

For horses with a photic trigger for headshaking, management strategies to control light exposure may lead to improvement of symptoms.

Some of these include: ^[3][4][5]

• Tinted contact lenses
• Face mask (Guardian mask)
• Topical sunscreen over muzzle and nares

However, UV light blocking strategies are often not effective when used alone. ^[7]

Adjunct Therapies

Bodywork such as acupuncture, electroacupuncture, craniosacral therapy, physiotherapy, and chiropractic adjustment may help some horses. However, research is lacking on many of these modalities. ^[6][7]

Nutritional Supplementation

Many horse owners have explored nutritional supplements as part of their approach to managing TMHS, with certain supplements showing potential for reducing symptoms.

Magnesium is an essential mineral involved in numerous physiological processes, including nerve and muscle function. Research suggests that magnesium supplementation may support horses with TMHS by modulating nerve activity and reducing hypersensitivity. ^[4][7]

Magnesium has been shown to influence nerve function by reducing catecholamine release, blocking neuromuscular calcium channels, and increasing the threshold for nerve firing. These mechanisms can help reduce nerve excitability and alleviate discomfort associated with neuropathic conditions. ^[4][7]

Scientific studies have identified a potential benefit of combining magnesium and boron for supporting normal nerve function in horses with headshaking behaviors. ^[7] In another study, 40% of owners reported improvement in headshaking following oral supplementation of 10-20 grams of magnesium daily. ^[4]

PENS Therapy

Percutaneous Electrical Nerve Stimulation (PENS) therapy is a technique commonly used to manage neuropathic pain in humans and may also offer benefits for horses with TMHS. ^[5]

In one study, an electrically conductive probe was inserted near the infraorbital nerve under ultrasonographic guidance to reduce nerve sensitivity through electrostimulation. This procedure provided short-term symptom relief for some horses. ^[5]

PENS therapy can be performed under standing sedation and carries minimal risk of side effects. Research indicates that remission of headshaking symptoms was achieved in just over half of the study horses, with an average remission period of 9.5 weeks. A second treatment often extended the duration of symptom remission compared to the initial treatment. ^[6]

Surgical Procedures

When all other treatments fail, interrupting signal transmission in the affected branch of the trigeminal nerve may be the only remaining option. Evidence supporting the involvement of the trigeminal nerve — such as improvement of symptoms with perineural maxillary nerve blocks — should be obtained before pursuing invasive procedures. ^[3]

Bilateral infraorbital neurectomy, performed at the point where the nerve exits the infraorbital foramen, was the first surgical procedure reported for TMHS. However, this approach worsened the condition in some horses and provided symptom relief in only a limited number of cases. ^[3]

In addition, some horses traumatized their nostrils following bilateral infraorbital neurectomy due to a lack of sensation or suspected hyperesthesia (extreme sensitivity). As a result, this procedure is no longer recommended unless repeated infraorbital perineurial anesthesia consistently demonstrates symptom improvement. ^[3]

The next surgical treatment developed was chemical sclerosis of the maxillary nerve. While this procedure provided symptom relief for some horses, headshaking behavior often recurred within a few months. ^[3]

More recently, compression of the maxillary nerve using platinum vascular embolization coils placed in the infraorbital canal near the maxillary foramen has been attempted to achieve long-term resolution of TMHS. Reported success rates after at least six months were 60%; with a second procedure, the success rate increased to 85%. ^[3]

Finally, for TMHS symptoms triggered exclusively during exercise, a permanent tracheostomy has been employed. Since increased airflow through the nasal passages during exercise can act as a stimulus for headshaking, this procedure has shown potential for long-term success in such cases. ^[3]

Prognosis

The prognosis for horses with TMHS depends on the underlying cause of the syndrome and varies significantly from horse to horse. Unfortunately, research indicates a progressive deterioration in outcomes for most horses affected by this condition. ^[8]

In cases where horses fail to respond to treatment and management strategies, they may become unsuitable for riding or other activities. For some horses experiencing severe, unmanageable pain, euthanasia may be considered the most humane option. ^[3][6]

Frequently Asked Questions

Here are some frequently asked questions about trigeminal-mediated headshaking in horses:

Summary

Trigeminal Mediated Equine Headshaking Syndrome (TMHS)
is a neuropathic condition causing involuntary, repetitive head movements in horses that are linked to trigeminal nerve dysfunction. It significantly impacts a horse's quality of life, with treatment success varying widely based on the individual and the approach.

• TMHS is thought to result from hypersensitivity or reduced activation thresholds of the trigeminal nerve, leading to pain triggered by light, touch, or airflow.
• Horses with TMHS typically display vertical headshaking, nasal irritation, snorting, or rubbing their muzzle, often worsening during exercise.
• Diagnosis involves clinical observation, nerve blocks, and advanced imaging (e.g., CT scans) to confirm trigeminal nerve involvement and rule out other causes.
• Management strategies include medications, nose nets, nutritional supplementation, UV blocking techniques, PENS therapy, or, in severe cases, surgical interventions.
• Outcomes vary, but TMHS often leads to progressive symptoms. Severe cases may render the horse unsuitable for use or require euthanasia as a humane option.